1. SPACE-BASED SOLAR SHIELD TO OFFSET GREENHOUSE EFFECT

- A thin glass W buflt &om lanarmaterIals and 1- near the h t Lagrange point of the --Sun sys- conld offset the greenhouse effects msed by the CO, buU&~p in the Earth's atmosphere.

T d d g tbe near planets of the solar system is a long, '

complex and ambiiious ufldertaHng lyhg welI lato Man's C futare. There are twomajmprobkms to be addressed viz global

t e m p m m a n d ~ ~ A w l g g e s t a d m e t b o d for the control of phmtary tempratures f the ufse of apace- based shields to modify the incident solar flux, T d o m h g shields for planets such as Venus or Manr would, of necessity, be large, cumpkx s irmms requhkg vast UUOUPQ of lunar or asteroidal material and a well estabhhed space mamd-actming and long-range lmmpWon system. One possible stepping- stone to u d a m n d b g and mastering the ~ o l o g i e s and physical processes involved would be the m-on of a _ shield to offset the &reenhouse effect on OUT own planet, Earth t Suchaproject wonldbemgch SmaIlerinshanddeandaot require interplanetary capabililimt. 1 ~onccm ha^ expressed w m ~ d ~ e for the &gins

f composition of the hrthls abmqhex In the twentielh an- Wl' almospWconcenmtion ofmokda suchas 1 CO,=8orb infrared radiation, has M e d fhe Imp- ping of thermal Wtia In the afmosphere now refetred to as the greenhouse effect. As the cotunlmtion of thesegases rises, t h e ~ ~ u l t i n g ~ ~ i n ~ a o d ~ c l i m a ~ ~ become more &ous. Because of the q l e x i t y of the many pracesm involved, there is mn~embleuncertainty regarding the rate of build-up of the greenhouse gases and the magnitude of the resulting climatic changes [I]. The time required for

t removal of these gases b m the ahnosphere by nabrsl proc- esses is also uncertain: current estimam are several centuries. The umeminties in the scale and M m of the clhadc

5 c h g a from the gteenhouse effects has M to ~~IIS ~mmorcnsearcbistothescpmblemsajwe~aJ~ctiomon

the generation of greenhouse gases, Themostimportant mtrk- lion would be on the huihg of h ~ b o m for power gmemtion and transportation Since the W - u p of CO, in the atmosphere is cumulative atid as there are no accepkd Whd- cal solutions to the greenhouse e&ct. it may prove crucial M restrict the generation of t b e gasw as soon as pos#ible, perhaps in the 1990s. Thc ' of a possible kcbnicd

( solution could thus have a m a ~ - b x m i m p a c t i n iufluenc- ing short term consumpdon reslrktions, even if the solution could not be implemented until the nexl century.

A concepmdy simplemetbod for offsetting the greenhouse radiation trapping effects would be to decrease the solar haling by the use of a space-based s o h shield 121. A p ~ ~ l y 2% of the solar radiation reaching Earth must l~ blocked to offset the p d ~ m d greenhouse trapping expected in tbe next cenhrry

+ [3]. The shield poshhted would be 20M3 Irm iu diameter and i

located1.5~ 106kmbn~arth,nkthefjrst~grangepoint between the Baah mithe Sun, A shield lop thick would weigh a m y 1O1I kg andmay cost&omone to ten WD ~ . I t w o u l d b e f a b r i c a t e d f r c w ! l u n a r ~ ~ e d b y . a m dtiver.

The space M d nmst be placed in an orbit where it remains positioned between the Earth aad the Sun. This point will be near the dassiwl first &I) p o h L The hation is detedned by two req ' u . F i y theangdxvelouty of ~shieldaroundthRSunandof~~mustbethesameso that~eshieM&inalinebetweenthe~smdtheSun. Secondly, thm must be an acceleratfon bafance on the shield between lk mtripital accekation h m tfae orbit around the Snn, thegraMonaI accelerations o f f h e m and the Sun arid the proton amkrzdjon on fie Meld A photon thtast of zero would 1- the shield exactIy at the L l point. Combining these requhmmta givea,

where m, M and G are the masses of the Eaah and Sun and the graviwnal cmstmt R and S are the disCanm from the Baah to ~ ~ u n a n d f r ~ m ~ ~ t o t f r e s h i e ~ d ~ p h o t o n ~ t i o n of the shield is a For zero photon th- one obtaias the L1 point

So= 1 .50~ 1 0 6 h

When fhm is a photon -011 on the shield, the balance pointbecomes

8 * z* 11 + (113) t q 1

~g 5 WZ2 = 0.0177 d s e 2

where % is the Earth's gravitational accelmfion at the shieM distance. The sbield orbit will be semi-stable as any small radial per-

turbalion towards or away from the Sun will cause the shield to be pulled out of position. H o w e v m , ~ a t i o n s peqendicular to the Ea&Sun axis will be stable. SWn-freeping at the Ll point requires constant adjustment to the orbit This is why there will lx no dust and natural satellites at the LI point, as commonly found at tfie L5 or Trojan orbits. The ~ t i o n s required to hold this orbit are very small and well within the capabililies of the shield.

The shield is balauced only at the IWh-Sun axis. AlI other ~ o f t h e s h i e l d a r e d r a w n m t h e a x i s b y a ~ ~ m at

where r is the rsldial dismce from the Eath-Snn axis. To be in an orbit around the Barth-Sun axis, &e cenlripital aademtion wm must balance the radial mlesation

If the shield is opaque, then the Sun side should have a I I ~aectivity (high absorptivity} for the solar sprrmm. %

photon tbmst h m radiated Wmed energy can be used to offsettbethxustfromabsorption. T h e ~ e m i s s i v i t y ~ h ~ ~ beminimised on the Sun side andmaximi& on the i An ideal opaque shkld would scatter the Barth bound solar i energyiotodiffUseinfrarsdenergy.

The shield may also h transparent and simply scatter the visible p h o m away from the Brtb. The required scmg I angle, 0, is 8.5m or one-balf degree. A #ass shield may act a prism to Msc t S w h t away lbm the planet in -ce with Snell's law. The light beans are d e f l m as they enwand exit the thin ghss shield If both surfaces of the glass are me] tbe deflections cancel and the light beams continue in their original k f i o a . If h e two s-s are at aa angle B with respect to each other, the glass then acts as a prism. Forthe near- n o d incidence w m o n used to - shie ldra. tioq tbe -on angle, 0, is equal to: --

where il is the augular veIocity of a section of the W d Since fl is independent of r, the shield can rotate as a solid body and have each section be in orbit around the Earh-Sun axis. Zn this condition there would be no stresses in the shield. A faster rotation rate than is required for gravitational balance will create a radial m a s in the shield, which may lx desirable to help maiotain the shield as a flat disc.

The disc rotation will, unfotlmatly, act as a gyrosmpe which keeps the disc orientated with its axis pointed in one direction. Since the disc axis must always point toward the Sun, a twque must be applied to tbe disc by a corn1 system to muse the disc to precess at one cycle per year. It is not clear if this control system is simpler than using solar sails at the wkmeter of the disc to supply a radial teision to balance h e radial gravitational acceferation.

3. PHOTON THRUST OF SHIELD

When a photon is absorbed or emitted, its momentum is tram- ferred to the shield. If &e photon is reflected, the momentum kansferred is twice the photon momenmu The solar pressure from incident radiation on the shield is hen

n = index of rehction - 1.0 for vacuum

rn 1.5 for most glasses

w h a and r are the shield absorptivity and reflectivity for sunlight. E and a, are the infrared emissivities of the shieId on . the Sun a d Earth sides. For a shield of tbkhess t and density p; the acceleration, a, of the shield is then

a = Plpt

Shield acceleration may be controlled by the optical design.

The de£ktiw angle dws not depend to first order on h e angle of incidence of ae hght on the shield so the orientation of the shield would not have to be cIoseIv oonfrolled.

The shieldmay have a oishaUow on one side and be flat on the other. If the distance from peak to valley is 200 microns, then the required change in t h i h e s s is 3.4 microns. If the average shidd thickness is 10 microns, this may be an aweptable change in thickness. If a thhex shield is &shed U?e parallel grooves must then be closer together.

A transparent shield would reflect some of the Zight at both the entering and exiting s-a. The amwnt reflected could be minhkad by appIying complex layeTed coatings to the glass but the relatively small bendt incurred may not justify the cost. Thereflectiviry would be 0.04foran uncoated surface.

T h e p h o t o n ~ a t i o n f o r a f Optransparentsbieldmade~f glass with a density of 2.5 is

The displaced orbit position is

4. SHIELD SIZE AND EFFECTIVE BLOCKAGE

The effective bl-e by the shield is given in Fig. 1 as a fundon of the shield diameter. For a shield diameter lest than 1200 Irm, the whole projection of the shield area on to the Sun's surface would lie within the Sun's disc when viewed fiwmaY point On the Earth's surface. The solar blockage would, thw- fore, be uniform overthe Earth's surfam. For a sbieId di-W- of 2000hh the projected W i d disc fa& partially off the Sun's disc only for Earth locations on the outer 6% rim of the projected Earth disc, i.e. of the standard circle map showing one side of the Eutl~. Except for arctic regions, these wil l be the regions near sunrise and sunset. Thus, the shading aver@ over the entin: day will be almost constant across the eartbwrth the axtic regions reaiving slightly less shading. This condition should avoid some of the potenw political probloblems asoci- ated with having some sections of arb shielded morethan others.

REFERENCES

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C

r: 0 ape m 1000 1m 1- 1600 i

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r ---I ~ [ 4 j * T b e ~ o f ~ ~ f r O m v ~ s l u ~ a r 5- ~oihrnnstfktbe&~ ' s e d . M o i t ~ w i l l k s ~ y

1, M.~andJ.Th,&Gmnbw~ctdSm~Riw;:VNRCn: m a d - &m~I;hh&cB+Sda,S,J.H-d NY; 1984 and H.W. Bemsrd; G r u b Em: B-* T.-&:--&~,W*DC.ISW 1980. ppI3@163.

2 MJ. Fogg, ThcTt-~ ofVarms~~40~551-564(1987) d R A . 4. W . ~ ~ ~ ~ ~ ~ ~ ~ o f h U 3 f - l w a n d him, Tdformine Mm d Vcuus Uning Maebirro ~ - n p ~ P b & q I~WUII, 1985. S ~ S W ~ ; ms36 139-1451 (r9a3y s. L ~ M ~ W ~ ~ 19as;ppm1.

3. J . f I a o s m , A . L a c y D . ~ O . ~ P . h , L ~ R ~ d J . 6. J. &, Whatt &Y@ halyria off* m ' 5 k ( C ~ W M AlAA hi@rmc#: AIAA; 1981.

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~ a r e n t h l O p ~ F o r a n o p a q w ~ a n a p p m priatecoatingmadwialwpaldbvetobefo~forthe~s s u ~ . l r o n m a y b e o b t a i a e d ~ l n n a r ~ w i t h o n I y moderakeffwthtisprobablymodemetouseasthebase mater fa l foranopaqmsh ie ldAny~nsedmustbe ~ p a b k o f m a i n ~ h ~ f 0 I f o r h I P ~ - ence. of radiation h solar wiad, solar W ~ W O O and cosmic rap.

T h e s h i e M ~ w o d d b e ~ s e d ~ g l a s s i n g o t s then dram into lbin &MS. Gha -011 kchnkpa must be InvdgaW to detemrine if 10p is a viable ?hkha. Commercialplasdcwrapis13ptlWand ' foilis *ally 13 to 25b Degigns for admmxd solar sails have been p r o p s e d - 2 p d m [5LTheglassmaybelaunched to tbe shield Iwation by amass drivet. A number o f s b d i a 161 h a v e ~ t e d t b a t m a s s d r i v ~ m f e a s i b L e a a d e c o ~ f o r lallmhingunmannedpaym hthe# lunar mface.Iftlle glasssheetis ~ y k x i b k i t m a y beformedintosbeeton d l e l ~ ~ a f t d ~ i o r o U s .

a1 citm ~~manyotherdes ignprob le .ms~as tbe fabr l ca - - tion of glasa she&, the'impct of lunar pdmbations and the m h r w required transfer orbits from the Moon to the shiefd which warag s h o u l d b e i a v a t & a t d . T h e ~ d ~ ~ t b e ~ -

hTollil tlonal mnlml system, the idmlmhm to build and maintain h e h theshieldandtheco11lzolafd~~ccharge aresomeofthe

~ i 2 e e m o f ~ major nndefhd systems. - ~ ~ a n d c o ~ o f t h i s ~ w o u l d h e n a r m a u s b u t ~ 5. SHIELD D&LGN e c o ~ [ ~ ~ t o f t h e ~ m s e e f f e c t m a y bemachlarger.

The scale of Ws project is not beyond ?he scale of man's Thesield~bab~wodbe~efrmnlm~als.Lunat d v M e 8 on Barth (Fig, 2) but it is unclear whether stsch a soil cah be formed into glass for either a iranqment m opaque m s i v d project could be m M e d in space.